1. Field of the Invention
The present invention relates to gold(I) and gold(III) N-heterocyclic carbene (NHC) complexes. More particularly, the present invention relates to synthesis of cationic gold(I) N-heterocyclic carbene (NHC) complexes and neutral gold(III) N-heterocyclic carbene (NHC) complexes.
2. General Background of the Invention
Although, historically, organogold complexes have been underutilized in organic synthesis, numerous publications have recently emphasized the beneficial role of gold(I) in catalysis.1 Organic transformations such as skeletal rearrangements (cycloisomerizations),2 carbene transfer reactions,3 indolizations,4 oxidations,5 and hydrosilylations,6 are examples of the diverse chemistry mediated by organogold catalysts. Such transformations have been achieved with low catalyst loading and high turnover numbers. The gold(I) center must have two coordination sites occupied to ensure stability of the complexes and thereby avoid reduction to gold(0).7 The most commonly employed ligands so far have been phosphines (PR3),8 and most recently N-heterocyclic carbenes (NHC).9 Both ligand families exhibit strong σ-donation and coordination of such ligands result in good stability of the Au(I) complexes towards air, moisture and thermolysis. It is interesting to note that gold has even a stronger affinity for N-heterocyclic carbene than for phosphine and other Fisher acyclic carbenes.10 A broad range of catalyzed transformations by inorganic gold(III) salts has been reported in the literature, examples include hydroaminations,11 [4+2] benzannulations,12 functionalization of aromatic C—H bonds,13 cycloisomerizations,14 and addition reactions to heterocycles.15 Most often AuX3 (X═Cl or Br) salts are directly used11-16 and only a limited number of examples of well-defined organogold(III) complexes acting as catalysts are known.17 No catalysis mediated by (PR3)— or (NHC)Au(III) complexes has been reported so far. This is quite surprising since the chemistry of the arsine,18 stilbine,19 phosphine,18b, 18c, 20 and carbene21 gold(III) complexes, was first examined in the mid-1970's. Since these initial studies only a limited number of publications have focused on this chemistry. Notable exceptions are the extensive studies performed on gold(III) phosphine complexes by Schmidbaur et al.22 Since then, C-tetrazolato,23 bis-thiazolinylidene and bis-(NHC) gold(III) complexes bearing carbene moieties, have been published.24 Nevertheless, no example of mono-(NHC)Au(III) complex has been reported, the mono-(4-methylthiazol-2-ylidene)AuCl3 being the closest related complex reported so far.24a 
For more information about the background of the present invention, see the papers entitled “Synthesis, isolation and characterization of cationic gold(I) N-heterocyclic carbene (NHC) complexes” Chem. Commun., 2006, 2045-2047, and “Synthesis, Characterization and Reactivity of N-Heterocyclic Carbene Gold(III) Complexes.” Organometallics, 2007, 26, 1376-1385, both attached to our U.S. Provisional Patent Application Ser. No. 60/910,385 and incorporated herein by reference.